1. Field of the Invention
The present invention relates to a mobile radio apparatus, such as a multi-mode portable communication terminal, for transmitting various types of radio signals and also relates to a radio unit in the mobile radio apparatus.
2. Description of the Related Art
Various communication systems, such as PCS (Personal Communication Services), the cellular system, AMPS (Advanced Mobile Phone Services), etc., generally use radio channels of different frequency bands.
Accordingly, multi-mode portable communication terminals need to transmit radio signals of different frequency bands. More specifically, a portable communication terminal compatible with two communication modes needs to transmit first and second communication mode radio signals of different frequency bands separately as shown in FIG. 3.
FIG. 4 is a block diagram illustrating the structure of a radio unit incorporated in a conventional dual-mode portable communication terminal.
As seen in the figure, the conventional portable communication terminal comprises power amplifiers 1 and 2, duplexers (DUP) 3 and 4, diplexer (DIP) 5, antenna 6, switching circuits 7 and 8, control section 9 and bias generating section 10.
The first transmitting signal for the first communication mode is amplified by the power amplifier 1, and then guided to the antenna 6 via the duplexer 3 and diplexer 5. Similarly, the second transmitting signal for the second communication mode is amplified by the power amplifier 2, and then guided to the antenna 6 via the duplexer 4 and diplexer 5. From the antenna 6, the first or second transmitting signal is transmitted by radio.
The two different communication modes are not simultaneously used; therefore the power amplifiers 1 and 2 do not need to simultaneously operate. This being so, the supply of power to the power amplifiers 1 and 2 is controlled by the switching circuits 7 and 8, respectively. The ON/OFF of the switching circuits 7 and 8 is controlled by the control section 9 so that only one of the power amplifiers that corresponds to the signal to be transmitted will operate.
When the power amplifiers 1 and 2 operate, they need a bias. This bias is generated by a bias generating section 10. Only when one of the power amplifiers 1 and 2 needs to operate, the bias generating section 10 generates a bias under the control of the control section 9.
In a conventional portable communication terminal having a plurality of power amplifiers as described above, respective switching circuits are provided to enable the power amplifiers to be turned on and off individually. This structure is employed to minimize the power supplied to the power amplifiers so as to suppress the current consumption.
To enable the power amplifiers to produce a large gain, a very large current needs to be supplied thereto. Switching circuits for turning on/off such large-current amplifiers therefore need a large capacity. Since the same number of large-capacity switching circuits as the number of communication modes is necessary, an extremely large space is needed to mount the circuits.